In a modern automobile the engine block is one of the heaviest single components. Making the block in a bimetallic manner, such as by fabricating it from an aluminium alloy and placing cast iron sleeves into the cylinder bores substantially reduces the weight of the block compared to conventional cast iron blocks. The aluminium alloy generally contains 5-10 wt-% Si as well as small amounts of other additions. The cast iron is generally grey cast iron but also pearlitic cast iron is used. The machining of a block as cast to final shape and dimension is generally made in transfer lines or flexible machining centres and the time pressure is high. An important step of the engine block manufacturing process is to provide the block with a flat upper surface for mating with the cylinder head. Often this operation is a bottleneck in the production. Machining of conventional unimetallic engine blocks (i.e. cast iron) is generally accomplished by common machining processes such as high speed milling utilizing ceramic inserts, such as silicon nitride, coated cemented carbide on the milling head. Although satisfactory when utilized for unimetallic blocks, this approach tends to produce undesirable results when used with blocks fabricated from two materials, one of which is soft, i.e., aluminium normally requires a rather high cutting speed, and the other of which is brittle, i.e., cast iron normally requires a lower cutting speed when coated cemented carbide is used. Thus, for machining of aluminium, polycrystalline diamond (PCD) is generally used. Such tools are relatively expensive, however, and wear rapidly in iron containing materials such as cast iron. Moreover, optimal milling for soft versus brittle materials is different. For example, most high-speed milling cutters made for softer materials, such as aluminium, operate most efficiently at substantially greater rake angles than those used for harder materials such as cast iron. Clearance angles, or the angle between the land and a tangent to the cutter from the tip of the tooth, also depend on the various work materials. Cast iron typically requires values of 4 to 7 degrees, whereas soft materials such as magnesium, aluminium, and brass are cut efficiently with clearance angles of 10 to 12 degrees.
When milling cutters with a close pitch are used for machining, there is a resultant change of about 30-40 inserts as they are worn out. One typical wear mechanism in the tool insert is a built up edge; however, this may lead to a bad surface finish and a failure of the cutting edge will lead to rapid wear of the insert. The main reason for multiple tool changes is the surface finish and the high demands for the surface finish, which leads to the frequent tool changes.
When wet milling is used, due to surface finish and chip evacuation requirements, emulsions used in machining may raise environmental concerns and potential health risks; thus leading to a higher cost.
EP-A-1335807 relates to a method of milling a material comprising aluminium and cast iron. By using a silicon nitride based cutting tool insert at a cutting speed of more than 1000 m/min, an unexpected increase in tool life has been obtained. However, not all transfer lines or flexible machining centres have speed capability >600 m/min.
EP-A-1205569 discloses coated milling inserts particularly useful for milling of grey cast iron, with or without cast skin, under wet conditions at low and moderate cutting speeds and milling of nodular cast iron and compacted graphite iron, with or without cast skin, under wet conditions at moderate cutting speeds. The inserts are characterized by a WC—Co cemented carbide with a low content of cubic carbides and a highly W-alloyed binder phase and a coating including an inner layer of TiCxNy with columnar grains followed by a layer of κ-Al2O3 and a top layer of TiN.